Some centrifugal pumps have “cantilevered” designs, wherein the impeller shaft is supported by bearings located on only one side of the impeller. Others are of a “between-bearing” design, whereby the impeller shaft is supported by bearings located on either side of the impeller. Often, the impeller shaft in a between-bearing pump is horizontal in orientation. Multistage centrifugal pumps are used extensively in high pressure pumping applications for which the pressure to be developed is greater than can be practically generated by a single impeller. For such multistage pumps, the inclusion of a plurality of pump impellers on a single shaft typically makes it necessary for the pump to be of a “between-bearing” design.
With reference to FIG. 1, a characteristic of between-bearing pumps 100 is that that the impeller drive shaft 3 penetrates the pressure casing 6 of the pump 100 and extends to atmosphere at two locations 102, one on either side of the pump 100. Leakage to atmosphere at these penetrations is typically controlled by mechanical shaft seals 4A, 4B on both sides of the pump 100, which effectively prevent objectionable quantities of pumped fluid from leaking to atmosphere. The bearings 8 are located beyond the seals 4A, 4B on either side. In some designs, the bearings 8 are surrounded by separate chambers 112 through which a lubricating and cooling fluid is circulated.
In most multistage pump designs the impellers 15 are arranged in series, whereby the low pressure inlet 104 is at a first end of the pump and the higher pressure discharge outlet 106 is at a second end of the pump. Thus, typically one or more mechanical shaft seals 4B at the second end must withstand a higher pressure than the shaft seals at the first end 4A, necessitating the use of seals of different designs and complexity within the same pump unit.
Some pumps must be able to handle slurries containing suspended solids which are abrasive. These abrasive solids are potentially damaging to the mechanical seals, which rely on highly polished surfaces separated by a micro-layer of fluid film to prevent wear and prevent leakage. It is well known within the art that fluids will flow along a path from high pressure to low pressure. To prevent a tendency for the abrasive solids in such cases to move across the seal faces from the high pressure internal region of the pump to atmosphere, a dual seal 108 is often used in conjunction with another clean fluid referred to as a barrier fluid.
A dual seal 108 typically includes two complete seal assemblies 4A, 4B mounted coaxially on a shaft adjacent to each other with a barrier fluid filling a space 110 between them, such that one seal is interposed between the pumped fluid and the barrier fluid and the other seal interposed between the barrier fluid and atmosphere. The barrier fluid 110 is maintained at a higher pressure than the pumped fluid, so as to maintain a positive flow of clean barrier fluid across the seal faces into the process.
Dual shaft seals 108 are also frequently used in applications for which the process fluid is highly toxic and/or flammable. The dual seal arrangement adds redundancy to the seal, thereby reducing the risk of a hazardous release to atmosphere. Nevertheless, seal failures do sometimes occur and product is sometimes released to the atmosphere, causing injury to life and property.
The inclusion of dual shaft seal assemblies 108 on a between-bearing pump 100 adds to the cost and complexity of the pump, in that it requires the use of two dual seal assemblies, one on each side of the pump, and therefore requires at least four seals in total.
What is needed, therefore, is a simplified sealing arrangement for a between-bearing centrifugal pump which allows abrasive slurries and highly toxic or flammable liquids to be pumped safely using fewer mechanical seals than the four seals traditionally required by a between-bearing pump with dual seal assemblies. It would be further desirable if the possibility of atmospheric release could be reduced by eliminating a leak path to atmosphere which is inherent in existing designs.